In an attempt to increase the efficiencies and performance of contemporary gas turbine engines generally, engineers have progressively pushed the engine environment to more extreme operating conditions. The harsh operating conditions of high temperature and pressure that are now frequently specified place increased demands on engine component-manufacturing technologies and new materials. Indeed the gradual improvement in engine design has come about in part due to the increased strength and durability of new materials that can withstand the operating conditions present in the modern gas turbine engine. With these changes in engine materials there has arisen a corresponding need to develop new repair and coating methods appropriate for such materials.
The turbine blade is one engine component that directly experiences severe engine conditions. Turbine blades are thus designed and manufactured to perform under repeated cycles of high stress and high temperature. An economic consequence of such a design criteria is that currently used turbine blades can be quite expensive. It is thus highly desirable to maintain turbine blades in service for as long as possible, and to return worn turbine blades to service, if possible, through acceptable repair procedures.
Turbine blades used in modern gas turbine engines are frequently castings from a class of materials known as superalloys. The superalloys include nickel-, cobalt- and iron-based alloys. In the cast form, turbine blades made from superalloys include many desirable elevated-temperature properties such as high strength and good environment resistance. Advantageously, the strength displayed by this material remains present even under stressful conditions, such as high temperature and high pressure, that are experienced during engine operation.
The superalloys are thus a preferred material for the construction of turbine blades and vanes. The high-strength superalloys are noted as precipitation hardening alloys. Nickel, alloyed with other element such as aluminum and titanium, develops high strength characteristics that are sustainable at high temperatures, the temperature range that engine designers now seek. The strength arises in part through the presence of a gamma prime (γ′) phase of material. One characteristic of the superalloys is the high degree of gamma prime in cast materials.
While the superalloys exhibit superior mechanical properties under high temperature and pressure conditions, they are subject to attack by chemical degradation. The gases at high temperature and pressure in the turbine engine can lead to hot corrosion and oxidation of the exposed superalloy substrates in turbine blades. Those turbine blades at the high pressure stages following the combustion stage of a gas turbine engine are particularly subject to this kind of erosion, and the portion of a turbine blade at the blade tip is even more subject to corrosion and oxidation as this area of the blade also experiences high pressure and temperature. Blade tips are also potential wear points. Corrosion and oxidation are both undesirable in that these processes can lead to the gradual erosion of blade tip material, which affects the dimensional characteristic of the blade as well as physical integrity. In order to protect superalloy turbine blades, a coating may be placed on both the airfoil surfaces, and the blade tip, to act as a barrier between the engine environment and the substrate material.
Among other materials, conventional MCrAlY coatings have been used as one kind of coating on turbine blades to resist corrosion and oxidation. In the conventional formulation of MCrAlY, M represents one of the metals Ni, Co, or Fe or alloys thereof. Cr, Al, and Y are the chemical symbols for Chromium, Aluminum, and Yttrium. Some conventional MCrAlY formulations are discussed in the following U.S. patents: U.S. Pat. Nos. 4,532,191; 4,246,323; and 3,676,085. Families of MCrAlY compositions are built around the Nickel, Cobalt, or Iron constituents. Thus the literature speaks of NiCrAlY, NiCoCrAlY, CoCrAlY, CoNiCrAlY, and so on.
The efficiency of gas turbine engines also depends in part on the ability to minimize the leakage of compressed air between the turbine blades and the shroud of the engine's turbine section. In order to minimize the gap between the turbine blade tips and the shroud, turbine blades often undergo a final rotor grinding before engine assembly. This grinding attempts to closely match the turbine blade size to the shroud diameter. However this machining process can result in the removal of the thin MCrAlY or other overlay coating (Pt-aliminide) on the turbine blade tip. When this occurs the bare blade alloy is directly exposed to the severe conditions of the engine environment. This exposure opens the blade to corrosion and/or oxidation that causes blade tip recession or failure. These are factors that potentially result in performance losses due to higher leakage of compressed air between the blade tips and the inner shroud. Further the corrosion and oxidation ultimately leads to erosion or wearing out of the turbine blade tips.
In conventional methods, MCrAlY is applied to a turbine blade as a coating layer through a thermal spray coating process like low pressure plasma spray (LPPS) or electron beam physical vapor deposition (EBPVD). In the thermal spray coating process the MCrAlY coating adheres to the surface of the substrate through mechanical bonding. The MCrAlY coating adheres to asperities previously fashioned onto the substrate surface. This process does not result in a metallurgical or chemical attachment of the MCrAlY material to the underlying substrate. This is described in U.S. Pat. No. 6,410,159.
Additionally, conventional methods of applying MCrAlY coatings have deposited a relatively thin MCrAlY layer, such 5-50 μm, as described in U.S. Pat. No. 6,149,389. Such a thin layer makes it possible for the grinding step to grind off the coating if, for example, the amount of grinding exceeds the depth of the coating in any particular area.
The option of throwing out worn turbine blades and replacing them with new ones is not an attractive alternative. The high pressure turbine (HPT) blades are expensive. A turbine blade made of superalloy can be quite costly to replace, and a single stage in a gas turbine engine may contain several dozen such blades. Moreover, a typical gas turbine engine can have multiple rows or stages of turbine blades. Consequently there is a strong financial need to find an acceptable repair or coating method for superalloy turbine blades.
Hence, there is a need for a turbine repair and coating method that addresses one or more of the above-noted drawbacks. Namely, a repair and coating method is needed that provides a strong bond between an MCrAlY protective layer and the turbine substrate, and/or a method that allows the deposit of MCrAlY onto a superalloy substrate such that sufficient MCrAlY layer still remains on the blade tip after subsequent grinding process and/or a modified MCrAlY composition that provides improved properties and durability, and/or a method that by virtue of the foregoing is therefore less costly as compared to the alternative of replacing worn turbine parts with new ones. The present invention addresses one or more of these needs.